Depending on welding methods and welding conditions, various flaws occur in a weld zone of a test object, such as a welded pipe. These flaws cause a quality deterioration of the weld zone. For this reason, the nondestructive inspection of weld zones is carried out using X-rays and ultrasonic waves.
X-ray inspection can easily detect spot flaws, such as pinholes and slag inclusions, and has been used in many inspections. However, X-ray inspection has problems of low inspection efficiency, high equipment cost, and the like. For this reason, in submerged arc welded (SAW) steel pipe, ultrasonic testing is first performed and then X-ray inspection is performed only on opposite pipe ends and areas where it is determined by ultrasonic testing that there are flaws.
On the other hand, ultrasonic testing is suitable for detecting planar flaws, such as crack flaws and lack of fusion, and is superior to X-ray inspection in terms of inspection efficiency and equipment cost. Ultrasonic testing is therefore adopted to examine the whole weld zone except opposite pipe ends.
As an example of a conventional ultrasonic testing method for weld zones, an online automatic testing method in the manufacturing process of SAW steel pipe will be summarized below. In conventional ultrasonic testing of SAW steel pipe, as described in Non-Patent Literature 1 (“Ultrasonic Testing Method for Welded Pipe”, Iron and Steel Institute of Japan, Feb. 22, 1999, pp. 60-62), a contrivance is made so that various types of flaws occurring in a weld zone can be detected without being overlooked. This contrivance is realized by arranging a plurality of ultrasonic probes for detecting longitudinal flaws (flaws extending in the direction of the weld line of a weld zone) and those for detecting transverse flaws (flaws orthogonal to the direction of the weld line of a weld zone) on each of the inner and outer surfaces of a pipe. Specifically, as shown in FIG. 1A, ultrasonic probes A1 and A2 for detecting longitudinal flaws on the inner surface of the pipe, ultrasonic probes B1 and B2 for detecting longitudinal flaws on the outer surface of the pipe, ultrasonic probes C1 and C2 for detecting transverse flaws on the inner surface of the pipe, and ultrasonic probes D1 and D2 for detecting transverse flaws on the outer surface of the pipe are arranged to perform ultrasonic testing.
By use of an eddy-current type or optical seam (weld line) detector and a seam tracking mechanism, the steel pipe is linearly transferred in the longitudinal direction while ensuring that the above-described plurality of ultrasonic probes can be constantly located in prescribed positions relative to a weld zone, whereby the whole weld zone is inspected.
However, ultrasonic testing by ultrasonic probes of general K-form arrangement as shown in FIG. 1A has a problem as described below. That is, for longitudinal flaws, it is possible to restrain the effects of the inclination of flaws from the radial direction of the steel pipe and the shape of flaws by performing ultrasonic testing from opposite sides, with a weld zone positioned therebetween. For transverse flaws, however, it is impossible to restrain the effects as described above because the ultrasonic testing of flaws on the inner and outer surfaces can be performed only in specific directions (the transfer direction of the steel pipe or the direction reverse to the transfer direction).
For this reason, in order to meet the requirements for inspection which have become increasingly severe in recent years, as shown in FIG. 1B, the trend is toward providing additional ultrasonic probes for detecting transverse flaws. Specifically, as shown in FIG. 1B, ultrasonic probes C3 and C4 for detecting transverse flaws on the inner surface of the pipe and ultrasonic probes D3 and D4 for detecting transverse flaws on the outer surface of the pipe are added. Incidentally, in the example shown in FIG. 1B, ultrasonic probes E1 and E2 for detecting longitudinal flaws are also added in order to increase the density of ultrasonic beams in the wall thickness direction of the pipe.
However, an increase in the number of ultrasonic probes to be arranged and the number of flaw detectors connected to each ultrasonic probe results in a steep rise in equipment cost. In addition, because it is necessary to set the distance between a weld zone and an ultrasonic probe, flaw detection sensitivity and the like for each ultrasonic probe, addition of ultrasonic probes poses a problem of long adjustment time, which is necessary until flaw detection becomes possible.
In the ultrasonic testing of transverse flaws by the ultrasonic probes of the arrangement shown in FIG. 1A and FIG. 1B, flaw detection is performed by using a pair of ultrasonic probes with a weld zone positioned therebetween (an ultrasonic echo transmitted by one ultrasonic probe is received by the other ultrasonic probe; for example, an ultrasonic echo transmitted by the ultrasonic probe C1 is received by the ultrasonic probe C2). For this reason, it is necessary to simultaneously adjust the positions of the pair of ultrasonic probes and testing conditions. It is difficult to simultaneously adjust the positions and the like of a pair of ultrasonic probes.
Furthermore, it is known that in the ultrasonic testing of transverse flaws by the ultrasonic probes of the arrangements shown in FIG. 1A and FIG. 1B, untested regions in pipe end portions are wide.
A method for solving problems as described above has been proposed, for example, by the present inventors in Patent Literature 1 (JP2002-22714A). This method is intended for detecting transverse flaws by transmitting and receiving ultrasonic waves in the longitudinal direction of a weld zone (in the direction of a weld line) with the aid of an ultrasonic probe arranged just above the weld zone (hereinafter, referred to as an above-bead probe).
However, in the angle-beam ultrasonic testing using this above-bead probe, the effective beam width relative to the bead width direction of a weld zone (in the direction orthogonal to a weld line) is narrow. For this reason, this technique has the problem that although it is possible to detect flaws present in the center of the bead width direction, flaws present at positions away from the center of the bead width direction tend to be overlooked, and hence the practical application of the angle-beam ultrasonic testing using this above-bead probe has not easily moved ahead.
The above-described “effective beam width” means the length of a range in which flaw echo intensity is not less than a prescribed intensity (for example, −3 dB when the maximum intensity is 0 dB) in the profile of the echo from a flaw (a flaw echo) which is obtained when an ultrasonic probe is scanned. In other words, so long as a flaw is present in this effective beam width, it is possible to detect the flaw in question at densities of not less than a prescribed intensity (for example, −3 dB) although the position of the flaw in question deviates from a position facing the center of the ultrasonic probe.
In the angle-beam ultrasonic testing using an above-bead probe, the narrow effective beam width relative to the bead width direction (an effective beam width obtained when an ultrasonic probe is scanned in the bead width direction) is caused by the beam shape of a weld zone. In other words, as illustrated as an example in FIG. 2, a bead (an excess weld metal) remains on the inner and outer surfaces of a weld zone and, therefore, it is difficult to simultaneously detect flaws present at positions different in the bead width direction of the weld zone.
FIGS. 3A to 3C show examples of the profile of flaw echo intensity obtained when an ultrasonic probe is scanned in the bead with direction of a weld zone. Specifically, FIGS. 3A and 3B show examples of the profile of echo intensity for a longitudinal hole B 1.6 mm in inside diameter worked in the center of the bead width direction of the weld zone (see FIG. 3C) and of longitudinal holes A and C 1.6 mm in inside diameter each worked at positions deviating±5 mm from the center of the bead width direction (see FIG. 3C). FIG. 3A shows an example of the profile in which the size of a transducer provided in the ultrasonic probe is 10×10 mm, and FIG. 3B shows an example of the profile in which the size of a transducer provided in the ultrasonic probe is 20×10 mm.
As shown in FIG. 3A, when the size of the transducer provided in the ultrasonic probe is 10×10 mm, the effective beam width (the length in the range of not less than −3 dB) for each of the longitudinal holes A to C is on the order of 4 mm. In this case, scanning positions of the ultrasonic probe (positions in the bead width direction) at which all of the longitudinal holes are capable of being detected, do not exist. From FIG. 3A it is apparent that, for example, when ultrasonic probes are arranged at the positions indicated by open arrows in FIG. 3A, compared to the echo intensity of the longitudinal hole B, the longitudinal hole C obtains an echo intensity of not more than −6 dB and the longitudinal hole A obtains an echo intensity of not more than −12 dB.
From FIG. 3A it is also apparent that a maximum value of echo intensity differs even when longitudinal holes have the same size.
On the other hand, as shown in FIG. 3B, when the size of the transducer provided in an ultrasonic probe is increased in the bead width direction to 20×10 mm, the effective beam width for each of the longitudinal holes A to C increases to the order of 15 mm. For this reason, the longitudinal holes B and C can be detected by arranging the ultrasonic probe at prescribed positions (for example, the positions indicated by open arrows in FIG. 3B). However, it is difficult to detect the longitudinal hole A because of the low echo intensity.
From FIG. 3B it is apparent that when the size of the transducer is increased, noises generated near the toe of a bead are amplified, resulting in a decrease in the S/N ratio of flaw signals. A concrete description will be given below. The areas near both ends of the abscissa of FIGS. 3A and 3B correspond to the positions of the bead toe. The echo intensity (noise caused by the shape of the bead toe) is on the order of −21 dB maximum in the areas near both ends of the abscissa of FIG. 3A, whereas in FIG. 3B, the eco intensity increases to the order of −13 dB maximum. From this it is apparent that the S/N ratio of flaw signals decreases.
As described above, after all, it is difficult for an ultrasonic probe provided with a single transducer to detect all flaws worked at different positions in the bead width direction. For this reason, it becomes necessary to use an ultrasonic probe provided with a plurality of transducers.
Examples of a technique for preventing flaws from being overlooked by using an ultrasonic probe in which a plurality of transducers are arranged include Patent Literature 2 (JP3674131B). In the technique described in Patent Literature 2, a plurality of transducers are arranged on a straight line, ultrasonic waves are transmitted and received by selecting a transducer group consisting of a given number of consecutive transducers from the plurality of transducers and a transducer group that has been selected is switched one after another. And this switching pitch is set to be equal to or smaller than the practical effective beam width of ultrasonic waves radiated from one selected transducer group.
Incidentally, the above-described “practical effective beam width” is defined as the width of a beam in which a level of 3 dB below a peak value of a sound field intensity in the middle part of an ultrasonic probe (paragraph 0005 of Patent Literature 2) can be assured.
However, the technique described in Patent Literature 2 has a problem as described below.
The profile of a flaw echo intensity obtained when an ultrasonic probe is scanned cannot be uniquely determined only from the profile of a sound field intensity, and the flaw shape in the scanning direction of the ultrasonic probe has a great influence.
FIG. 4 shows examples of the profile of flaw echo intensity obtained when the same ultrasonic probe is scanned in the axial direction of a steel pipe for axial flaws (flaws extending in the axial direction of the steel pipe) and circumferential flaws (flaws extending in the circumferential direction of the steel pipe) worked in the steel pipe.
Because in the above-described examples the same ultrasonic probe is used, the profile of sound field intensity is the same, but as shown in FIG. 4, when flaws differ, the profile of flaw echo intensity becomes different. For this reason, in the practical effective beam width derived from a profile of sound field intensity, it is impossible to correctly determine the above-described switching pitch and flaws might be overlooked.